The present invention is directed to the frequency stabilization of a semiconductor laser, for example, a semiconductor laser diode. Frequency stability can be increased by a stabilization of both the injection current as well as the crystal temperature and is enhanced so that it is adequate for many applications. However, the remaining frequency fluctuations can amount to a few 100 MHz. When high demands are made of frequency consistancy, then one must fall back to stabilization with the assistance of external, frequency-stabilized resonators.
Ring resonators that can achieve extremely high quality can be realized upon employment of monomode optical fibers, as disclosed by Optics Letters, Vol. 7, June 1982, pages 288-290. The transmission and reflective behavior of the ring resonator is very similar to that of a Fabry-Perot resonator so that the fiber ring resonator, among other things, can be utilized in order to stabilize the emission frequency of semiconductor laser diodes, as disclosed, for example, from Proc. of IOOC-ECOC 1985, pages 833-836. Since, however, the refractive index of the optical fiber exhibits a great temperature dependency, fiber resonators react to temperature fluctuations with great modifications of the resonant frequency, and this will also involve a modification of the free spectral range that is defined by the frequency spacing between two neighboring resonant frequencies of the resonator and is equal to c/nl, wherein c is the speed of light in a vacuum, n is a refractive index of the one, annularly closed, optical waveguide of the resonator, and 1 is the length of the waveguide. The frequency stability of the laser diode stabilized in this fashion is not adequate for many applications, for example, for optical heterodyne detection, so that additional stabilization measures must be undertaken.
The stabilization of the free spectral range and, thus, of the resonant frequency of an optical resonator was heretofore achieved by the stabilization of the resonator length. This is usually achieved by realizing a temperature-compensating structure that uses invar rods or silica glass rods. However frequency stability in a fiber ring resonator is essentially defined by relatively great temperature dependency of the refractive index of the fiber. Operating such a resonator at a constant ambient temperature has hitherto been the only known possiblity of frequency stabilization.